Influenza A and respiratory syncytial virus ts mutants were evaluated in vitro and in experimental animals for evidence of genetic stability and antigenicity. A double lesion ts recombinant of influena A virus was produced which contained 2 ts lesions, each of which was relatively stable as regards reversion to wild type properties. This double lesioned recombinant (ts-1A2) was more restricted in growth in both the upper and lower portions of the hamster's respiratory tract than the ts-1(E) recombinants. Ts-1A2 was also more stable genetically in vivo than ts1(E) recombinants. Additional ts mutants were produced with the result that ts mutations involving each of the influenza A virus genes are available for study. The nature of the defect of the ts-2 mutant of RS virus was elucidated. This mutant is defective in adsorption-penetration, probably as a result of a defect in the surface glycoprotein required for penetration of the host cell. Ts-2 was completely attenuated for the chimpanzee, whereas other mutants such as ts-1, ts-1 NG-1, ts-1 NG-16 retained some virulence. BIBLIOGRAPHIC REFERENCES: Spring, S.B., Maassab, H.F., Kendal, A.P., Murphy, B.R. and Chanock, R.M.: Cold-adapted variants of influenza virus A. I. Comparison of the genetic properties of ts mutants and five cold-adapted variants of influenza virus A. Virology 77: 337-343, 1977. Chanock, R.M., Murphy, B.R., Spring, S.B., Richman, D.D.: Rationale for development of live virus vaccines by use of temperature sensitive mutants. In Younger, J.S. (Ed.) Microbiology, Washington, D.C., American Society for Microbiology, 1977, pp. 516-520.